The field of the invention is positron emission tomography (PET) scanners, and particularly PET scanners with retractable septa.
Positrons are positively charged electrons which are emitted by radionuclides that have been prepared using a cyclotron or other device. These are employed as radioactive tracers called xe2x80x9cradiopharmaceuticalsxe2x80x9d by incorporating them into substances, such as glucose or carbon dioxide. The radiopharmaceuticals are injected in the patient and become involved in such processes as blood flow, fatty acid, glucose metabolism, and protein synthesis. As the radionuclides decay, they emit positrons. The positrons travel a very short distance before they encounter an electron, and when this occurs, they are annihilated and converted into two photons, or gamma rays. This annihilation is characterized by two features which are pertinent to PET scannersxe2x80x94each gamma ray has an energy of 511 keV and the two gamma rays are directed in nearly opposite directions. An image is created by determining the number of such annihilations at each location within the field of view.
A typical PET scanner is cylindrical and includes a detector ring assembly composed of rings of detectors which encircle the patient and which convert the energy of each 511 keV photon into a flash of light that is sensed by a photomultiplier tube (PMT). Coincidence detection circuits connect to the detectors and record only those photons which are detected simultaneously by detectors located on opposite sides of the patient. The number of such simultaneous events (coincidence events) indicates the number of positron annihilations that occurred along a line joining the two opposing detectors. During an acquisition, coincidence events are recorded to indicate the number of annihilations along lines joining pairs of detectors in the detector ring. These numbers are employed to reconstruct an image using well-known computed tomography techniques.
When originally developed, PET scanners were strictly multiplanar scanners. In such PET scanners, each detector ring is configured to only detect annihilations occurring within (i.e., within the plane of that respective ring alone, and not to detect annihilations occurring at other positions within the PET scanner (i.e., annihilations occurring within the other rings of the PET scanner). Because each detector within each detector ring is capable of receiving photons coming in toward the detector from a variety of angles (rather than merely coming in toward the detector from the center of the ring of which the detector is a part), fixed slice septa are positioned in between each of the detector rings of the PET scanners. The septa, which are commonly composed of lead or tungsten alloy, shield the detectors of each individual detector ring from photons that have not originated from annihilations within the ring. The septa further have the function of shielding the detectors of the detector rings from scattered photons or other photons that are not resulting from annihilations (i.e., photons entering at either end of the cylindrical PET scanner).
FIG. 1 (Prior Art) shows a cross-sectional view of the detector ring assembly of one embodiment of a conventional multiplanar PET scanner having septa positioned between each of the detector rings of the detector ring assembly. In this embodiment, the septa allow each detector of a given detector ring to receive photons that are moving toward the detector from directions within a certain limited angle xcex1 outside the plane of the respective detector ring. Further in this embodiment, angle xcex1 is set sufficiently large that detectors of adjacent detector rings that are opposite one another are capable of receiving photons resulting from annihilations that have occurred in between the adjacent rings. Coincidence events that are recorded between detectors of adjacent detector rings are treated as though the precipitating annihilations occurred at positions exactly in between the adjacent rings, within cross planes formed at the junctions of adjacent rings (at the same levels as the septa). However, the septa prevent each detector of a given detector ring from receiving photons that are moving toward the detector from a direction beyond the angle xcex1. Thus, the image information provided by the detectors of each given detector ring is effectively independent of that provided by the detectors of the other rings, and concerns only annihilations occurring within that given ring (or the rings on either side of the given ring).
A major innovation in PET scanners that occurred in the late 1980s and early 1990s was the development of volumetric, or true-3D, PET scanners. In contrast to multiplanar scanners and as shown in FIG. 2 (Prior Art), volumetric PET scanners have no septa and consequently the detectors of each detector ring of the scanners can receive photons from a wider range of angles with respect to the plane of the respective ring than in multiplanar PET scanners. Volumetric PET scanners became feasible partly as a result of the increased speed of computers generally, since volumetric PET imaging requires determining the existence of, and processing information related to, coincidence events that occur not merely between pairs of detectors positioned on individual (or adjacent) detector rings, but also between pairs of detectors positioned on detector rings that are spaced more than one ring apart. Volumetric PET scanners allow for increased sensitivity relative to multiplanar scanners, since more coincidence events can be recorded. However, volumetric PET scanners also admit more scattered and random coincidence events to the data set from which the image is reconstructed than multiplanar PET scanners.
Most medium-end and high-end PET scanners available on the market today, including the GE Advance PET scanner manufactured by General Electric Company of Waukesha, Wis., have septa which are automatically retractable. Through the use of such automatically retractable septa, the PET scanners are able to operate as volumetric PET scanners (with the septa retracted) when the benefits associated with increased sensitivity due to volumetric PET scanning outweigh the loss in data quality resulting from the detection of more scattered and random coincidence events. This is typically the-case, for example, when the PET scanners are used for brain imaging purposes. However, the PET scanners are also able to operate as multiplanar PET scanners (with the septa extended) when the loss of data quality due to the detection of scattered and random coincidence events becomes excessive. This is typically the case, for example, when the PET scanners are used for body imaging purposes. Thus, PET scanners with automatically retractable septa are xe2x80x9chybridxe2x80x9d PET scanners in that the PET scanners can operate both as multiplanar and volumetric PET scanners depending upon the positioning of the septa.
Recently, the continued development of PET scanners has included the development of smaller sensing crystals within the detectors, which has in turn led to the use of smaller detectors and therefore to the use of detector rings having decreased width. By employing ever-smaller crystals and detectors, PET scanners have improved sampling and resolution, and thus are able to generate more accurate, higher-resolution images. For hybrid PET scanners operating in volumetric mode (and volumetric PET scanners), the use of smaller crystals and detectors does not reduce system sensitivity. Approximately the same number of coincidence events is detected regardless of the size of the crystals and detectors, although the number of coincidence events detected by any given crystal/detector decreases as the crystal/detector size decreases.
However, for hybrid PET scanners operating in multiplanar mode (and multiplanar PET scanners), the use of smaller crystals and detectors does reduce system sensitivity. As the size of the crystals/detectors decreases and the width of the detector rings of the PET scanner decreases, the number of septa separating detector rings increases (since the number of rings of the PET scanner must increase). Additionally, the range of angles (xcex1) outside the plane of a given detector ring from which a given detector on the ring is allowed to receive photons also decreases, since it is normally desired that coincidence events at most be detectable by adjacent rings.
For these reasons, a decrease in crystal/detector size (and detector ring width) by a factor of f leads to a loss of sensitivity per ring by a factor of f2. For the PET scanner as a whole (employing all of its detector rings), this reduced sensitivity per detector ring is partially offset by an increase in the number of rings by a factor of f. Even so, the overall sensitivity of the PET scanner is still reduced by a factor of f. Therefore, as the size of the crystals/detectors employed in a hybrid PET scanner decreases, the sensitivity of the PET scanner when operating in multiplanar mode decreases.
It would therefore be advantageous if a system was developed that allowed the implementation of reduced-size crystals/detectors within a hybrid PET scanner to provide increased sampling and resolution during operation in volumetric mode, and at the same time allowed the hybrid PET scanner to maintain adequate sensitivity during operation in the multiplanar mode.
It has been discovered that the decrease in sensitivity of a hybrid PET scanner when operating in the multiplanar mode due to the use of reduced-size crystals/detectors can be ameliorated if septa are placed not between successive detector rings, but rather are placed only between groups of rings of detectors.
The present invention relates to a PET scanner having a gantry, a plurality of sets of detectors supported by the gantry, and a plurality of septa that are supported by the gantry and are constructed of material which blocks photons. The detectors in each set are disposed in a plane and positioned around a central axis that intersects the plane, and the plurality of sets of detectors are spaced along the central axis. The septa are spaced along the central axis to separate groups of detector sets and block external photons from reaching the detectors. The PET scanner further includes a processor means for receiving signals produced by the detectors and indicating annihilation events occurring within a central region around the central axis; and reconstructing an image from indicated annihilation events.
The present invention additionally relates to a hybrid PET scanner having a gantry and a plurality of sets of detectors supported by the gantry. The detectors in each set are disposed in a plane and positioned around a central axis that intersects the plane, and the plurality of sets of detectors are spaced along the central axis. The hybrid PET scanner further includes a means for receiving signals produced by the detectors and indicating annihilation events occurring within a central region around the central axis; and reconstructing an image from indicated annihilation events. The hybrid PET scanner additionally includes a plurality of septa that are supported by the gantry and are constructed of material which blocks photons. The septa are spaced along the central axis to separate groups of detector sets and to block external photons from reaching the detectors. The septa are circular rings that are concentric about the central axis, and the septa are retractable along the central axis toward one end of the gantry.